The air in an air brake system is the "fluid" to activate the brake, much like brake fluid activates the brakes in your car. One difference is that you are not pushing air down the lines when you step on the brake pedal, instead you are releasing a metered amount of air, or "application pressure" from the reservoir, through the treadle valve (brake pedal/valve) to the brake chambers, to apply the brakes. Air also releases the parking brake, as the parking brake is always in the on mode, until you push a valve on the dashboard, sending compressed air to the parking brake chambers, releasing them. The parking brakes are spring powered, and the air over rides these springs.
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